Various mixing apparatuses are known in the art. For instance, a mixing apparatus has been disclosed in EP 0542 713. The mixing of fluids takes place in a mainly cylindrical mixing vessel by means of a revolving mixing element disposed therein. Homogeneity of a settling fluid in the mixing vessel is maintained by keeping a flow direction in the vessel as vertical as possible. This is ensured by an agitator shaft having attached thereto a first mixing element in the form of paddles or blades by means of which the flow in the vessel is kept columnar at the central section, as well as a second mixing element, also in the form of paddles or blades, which deflects the axial flow via a radial flow into a rising annular flow.
However, the device of EP 0542 713 is not suited for mixing viscous fluids as the attack area of the blades provides a too large flow resistance for an effective use in viscous fluids.
U.S. Pat. No. 5,246,289 discloses an agitator assembly for use in effecting dispersion of a fluid such as a gas in a liquid comprises a rotor having a rotatably driven shaft mounting a series of scoop-shaped blades which are oriented with the mouths of the scoops presented in the direction of rotation of the shaft, each blade being mounted at an angle of attack such that one end of the blade leads the other in the direction of rotation. To eliminate gas cavity formation, each blade is of generally streamlined configuration in section and the ends thereof are generally parallel to the direction of motion of the blade.
However, the device of U.S. Pat. No. 5,246,289 is designed for facilitating dispersion of a fluid in a liquid and avoiding gas cavity formation. The device is not suited for mixing viscous fluids because high rotational resistance of the disclosed device makes it unsuited for mixing of viscous liquids.
In U.S. Pat. No. 5,037,209 an apparatus for the mixing of fluids, in particular pasty media and a process for its operation, is disclosed. A stirring mechanism, with a plurality of hollow, at least partially conically shaped stirring elements, which are provided with two openings, are symmetrically offset and are fixed on the stirring shaft at least approximately tangential to an imaginary circular cylinder coaxial to the stirring shaft.
However, the conically shaped stirring elements of the apparatus disclosed in U.S. Pat. No. 5,037,209 have a relatively large attack area, leading to a rotation of the fluid in the mixing receptacle around the stirring shaft. A rotation of the fluid implies a less effective mixing effect as the relative speed between the conically shaped stirring elements and the fluid decreases.
In the case of certain media having a high viscosity, or a high load of heavy particles, there is the risk that a plug forms within the conically shaped stirring elements of the apparatus disclosed in U.S. Pat. No. 5,037,209, so that medium can no longer flow through the mixing element.
Moreover, manufacturing of the conically shaped stirring elements of the apparatus disclosed in U.S. Pat. No. 5,037,209 is costly. A metal sheet has to be rolled to the desired conical shape and the junction is welded. Manual treatment is required to give the conical element a desired smooth surface, especially in the region of the welded junction, such that viscous medium does not attach to the stirring element during operation.
Furthermore, it is difficult to provide stirring of the lower bottom layer in the mixing container by means of the conically shaped stirring elements of the apparatus disclosed in U.S. Pat. No. 5,037,209. For instance, in downward oriented dome shaped bottoms of such containers, bottom sediment is built up, at least in the center of the dome, during mixing with the apparatus of U.S. Pat. No. 5,037,209.
In addition, the stirring mechanism is difficult to assemble in the mixing container, where only a small inspection door is provided for access to the interior of the mixing container. This is especially the case where the stirring mechanism is provided in a non-symmetrical form, such as with three or more arms.
In practice, various products are sequentially manufactured in one and the same mixing apparatus. Between different products the receptacle, in which the products are mixed, has to be cleaned thoroughly in order to prevent cross contaminations. The receptacle is basically a closed container in order to prevent contaminants from entering the container during mixing. Also, during operation, for safety reasons, stirring apparatuses are designed to close seal-tightly. Still, cleaning is desired to be made as fast and easy as possible. One established method is high pressure cleaning, wherein a hose, having a spray ball at its end introduced into the tank for cleaning, is entered into the receptacle through a small inspection door, that can be opened for this purpose, in the top of the receptacle. The conically shaped stirring elements of the apparatus disclosed in U.S. Pat. No. 5,037,209 are difficult to clean. High pressure cleaning does not reach parts of the cones. Hence, the receptacle has to be filled with a cleaning liquid to a level above the stirring mechanism. Then a time consuming stirring of the cleaning liquid is performed. Cleaning time is further extended by a counter flow that is created in the cleaning liquid around the conical stirring elements. The counter flow deteriorates for instance the cleaning effect of the cleaning liquid at the frontal edge of the conical stirring elements. Moreover, a considerable amount of cleaning liquid, in the range of several thousand liters, is used for each cleaning process. Compared with e.g. high pressure cleaning by means of a spray ball, this leads to increased operation costs and environmental drawbacks, especially when the same receptacle is used for different products and changes are frequent, e.g. several times a day.
In U.S. Pat. No. 6,250,797 an apparatus for the mixing of fluids, in particular gas-to-liquid or liquid-to-liquid dispersion and a process for its operation, is disclosed. A impeller mechanism, with a plurality of blades, which have slots extending essentially all the way between tip and hub ends thereof, are symmetrically offset and are fixed on the stirring shaft at least approximately tangential to an imaginary circular cylinder coaxial to the stirring shaft.
Moreover, the slots are to ensure passageways through the impeller blades and thereby reducing the tendency for bubbles to grow or coalesce into large bubbles disrupting the mass transfer to the liquid which is pumped with the impeller. Furthermore, to achieve an efficient mass transfer in the liquid the impeller blades are inclined with a large slant angel in rotation direction and there each blade surface is substantially large.
However, the large blade surface area together with the slant angel give rise to a large energy consumption for the impeller system disclosed in U.S. Pat. No. 6,250,797, when applying it to a flowable medium with high viscosity, as for instance gruel or pap, due to large shear forces as the impeller system rotates.
Furthermore, the slant angel, with which the blades are arranged makes cleaning potential difficult as part of the blades are not reachable using high pressure cleaning, although the slots may ease the cleaning process they substantially add hidden areas with potential growth of bacterial. Altogether, the mixing system disclosed in U.S. Pat. No. 6,250,797 is not suitable for mixing food or liquid to be served to human or animal.
Moreover, with a flowable medium with a high order of viscosity the medium tends to follow in the circumferential direction of the rotating blades instead of being mixed in a vertical direction induced by the blades slant angel. Thus, the stirring effect of the flowable medium is absent or at least substantially reduced. Also, stirring efficiency is very low due to this fact, i.e. the amount of energy needed for an effective stirring is high in relation to the stirring effect obtained. Most energy is used for rotating the viscous media without obtaining a stirring effect.
In US 2002/0031048 a vertical mixer is disclosed with an up-ward conveying mixing spiral achieved with several mixing blades arranged after each other in the circumferential direction. It is further disclosed that an additional mixing spiral can be arranged in axial direction after the first mixing spiral with a transition zone arranged in between. Although an effective mixing is achieved the total number of individual blades needed can have an undesired effect on the viscous flowable medium, especially if it contains for example soft particles, such as fruits or other brittle substances. Moreover, as the specification in US 2002/0031048 teaches of multiple sets of mixing spirals, each comprising several mixing blades, question can be raised whether sufficient cleaning can be achieved, crucial if the mixed food or liquid is to be served to human or animal. The proposed construction of the mixing spirals and arranged mixing blades implies possible hidden areas which could promote a growth of bacteria due to insufficient cleaning possible without complete disassembly of the apparatus for cleaning.
Hence, an improved mixing apparatus for viscous liquids would be advantageous and in particular a mixing apparatus for viscous liquids allowing for increased cost-effectiveness, and/or mixing efficiency and/or ease of cleaning would be advantageous.